Chapter 13: Equilibrium and Human Movement

1. Chapter 13: Equilibrium andHuman Movement Basic Biomechanics, 4th edition Susan J. Hall Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University

2. Objectives • Define torque, quantify resultant torques, and identify the factors that affect resultant joint torques • Identify the mechanical advantages associated with the different classes of levers and explain the concept of leverage within the human body • Solve basic quantitative problems using the equations of static equilibrium • Define center of gravity and explain the significance of center of gravity location in the human body • Explain how mechanical factors affect the body/s stability

3. EquilibriumTorque Torque: • T = Fd Moment arm: • In the body, moment arm of muscle is the perpendicular distance between muscle's line pull and joint center • Largest moment arm at an angle of pull ~900 • Vector quantity, magnitude and direction • Fd & counterclockwise (+) & clockwise (-)

4. Resultant Joint Torques • Product of muscle tension and muscle moment arm produces a torque at the joint crossed by the muscle • Agonist and antagonist muscle groups • Net joint torque • Concentric and eccentric • Two joint muscles • Factors that affect net joint torques • Speed’s affect on net joint torques

5. Levers Lever: Fulcrum: First class lever: Second class lever: Third class level: Most levers within the body are third class

6. Lever Systems • Moment arm of applied force > moment arm of resistance • Resistance arm is longer than force arm Mechanical advantage = Moment arm (force) Moment arm (resistance)

7. Anatomical Levers • In the human body, most lever systems are third class • Arrangement promotes • Range of motion • Angular speed • Forces generated must be in excess of the resistance force • Two components of muscular force • rotary and parallel component

8. Equations of Static Equilibrium Equilibrium: • Three conditions for equilibrium: 1. Fv = 0 2. Fh = 0 3. T = 0

9. Equations of Dynamic Equilibrium Dynamic equilibrium: • Fx - māx = 0 • Fy - māy = 0 • TG - ī = 0

10. Center of Gravity (CG)Center of Mass Center of Mass / Center of Gravity: • The CG of a symmetrical object of homogeneous density, is the exact center of the object • When mass in is not constant, CG shifts in the direction of greater mass.

11. Locating the Center of Gravity • For one-segment, balance point in three different planes • As projectile, the body’s CG follows a parabolic trajectory • Weight vector acts as the CG

12. Locating the Human BodyCenter of Gravity Reaction board: • requires a scale, a platform & rigid board with sharp supports on either end. Segmental method: • uses data for average locations of individual body segments CGs as related to a percentage of segment length

13. Stability and Balance Stability: • Factors that affect: • Mass, friction, center of gravity & base of support Balance: • Foot position affects standing balance

14. Summary • A muscle develops tension and produces torque at the joint that it crosses. • Muscle and bones function as levers. • The angle of muscle pull on a bone produces rotary and parallel components of force • When a body is motionless, it is in static equilibrium. • The behavior of a body is greatly influenced by location of center of gravity. • Stability is resistance to disruption og equilibrium